1. Field of the Invention
The present invention relates to an improved electron beam collector, and more particularly, to a multistage depressed collector capable of efficiently recovering the energy of a large orbit electron beam.
2. Description of Related Art
It is well known in the art to utilize a linear beam device such as a traveling wave tube (TWT), klystron, or coupled cavity tube to produce microwave power. In a linear beam device, an electron beam originating from an electron gun is caused to propagate through a tunnel or a drift tube generally containing an RF interaction structure. At the end of its travel, the electron beam is deposited within a collector or beam dump which effectively captures the spent electron beam. The beam is generally focused by magnetic or electrostatic fields in the interaction structure of the device in order for it to be effectively transported from the electron gun to the collector without loss to the interaction structure. An RF wave can be made to propagate through cavities within the interaction structure and interact with the electron beam which gives up energy to the propagating wave. Thus, the microwave device may be used as an amplifier for increasing the power of a microwave signal.
In one particular type of electron gun, a hollow electron beam is formed. By varying the axial magnetic field, the electrons in the hollow beam can be made to orbit some of the magnetic flux lines. As the magnetic field is increased, a significant fraction of the axial energy of the electron beam is converted to motion transverse to the beam axis. This gyrating beam is used in several types of microwave devices which convert the transverse energy of the beam into RF energy. Examples of these devices are the peniotron, gyrotron, gyroBWO, gyroTWT, etc. A prior art gyrotron is shown in FIG. 1.
The hollow beam can be characterized as either a large orbit beam in which the electrons spiral about a guiding center of the beam near the axis of the microwave device, or a small orbit beam in which the electrons orbit around individual flux lines of the guiding magnetic field in the interaction region. The rotation of the electrons in a large orbit beam is induced by a magnetic field reversal at the front end of the interaction region. A large orbit beam is shown graphically in FIG. 2.
To operate the microwave device efficiently, the spent electrons which pass through the interaction region must be collected and returned to the voltage source. Any remaining energy in the electrons is released in the form of heat when they strike a stationery element, such as the walls of the collector. One type of collector, known as a depressed collector, is operated at a negative voltage with respect to the body of the microwave device. By operating at a depressed state, the electric field within the collector slows the moving electrons so that the electrons can be collected at a reduced velocity. This method increases the electrical efficiency of the RF device as well as reducing undesirable heat generation within the collector. Depressed collectors are discussed in U.S. Pat. No. 4,794,303, by Hechtel et al., which is assigned to the same assignee as the present invention, and which is incorporated herein by reference.
A depressed collector typically comprises a cylindrical structure having a plurality of electrodes arranged in stages. Each electrode stage of the collector has a negative voltage of increasing potential, such that the first stage has the lowest potential and the final stage has the greatest potential. In a linear beam device, the electrons with the lowest axial energy would be collected on the first stage electrode, while the electrons having the highest axial energy would travel to the latter stages for collection. The electrode potentials of a multi-stage depressed collectors are selected for efficient collection of a maximum amount of beam current at the lowest incident velocity.
However, a large orbit gyrating electron beam can not be directly collected by a typical multi-stage depressed collector. In a gyrating beam, the ratio of transverse velocity to axial velocity of the beam, known as .alpha., is usually between 1 and 2. Since increasing .alpha. will raise the efficiency of the microwave device, it is common for such devices to be operated at the highest .alpha. until a point is reached in which the device becomes unstable. Accordingly, high .alpha. electron beams have a majority of their kinetic energy in the transverse direction. Once the gyrating beam exits the interaction region and enters the collector, the absence of a controlling magnetic field causes the electron beam to expand rapidly. Consequently, the unguided beam impacts a generally thin cylindrical portion of the collector. In so doing, a majority of the electrons would be collected on the first stage of the multi-stage collector, significantly reducing the overall efficiency of the microwave device.
A solution which enables a typical multi-stage depressed collector to be used with a gyrating beam involves conversion of the beam's transverse velocity into axial velocity. This can be accomplished by adding a second magnetic field reversal at the end of the interaction region adjacent to the collector. By reversing the magnetic field, the rotation of the electrons in the beam about the guiding center is effectively terminated. The linearized beam can then be collected in a multi-stage depressed collector in the usual manner.
In practice, this method reduces the efficiency of the microwave device. Additional energy must be devoted to the magnetic field reversal. In addition, the overall device length must be increased to take into account the axial length of the magnetic field reversal region, and the increased axial length of the multi-stage depressed collector.
Thus, there is a need to provide a multi-stage depressed collector for use with a large orbit gyrating electron beam in which the electrons can be sorted and collected on the basis of their transverse rotational energy, and which does not require a second magnetic field reversal to linearize the beam. It would also be desirable if such a collector would have generally reduced axial length over conventional multi-stage depressed collectors.